Can the browser-tab icon be the symbol on its own, without the wordmark?

Yes: in a browser tab, an app icon or a profile image the symbol stands alone, in sand on petrol.

A tab has no room for the wordmark, and where the wordmark fits, the full logo is used. At 16px the three rings cannot be told apart, so the tab icon is a variant generated from the mark rather than redrawn by hand.